Most any good-quality duffel will serve this purpose. I'm a big fan of the Adventure Duffels that L.L. Bean makes. Very sturdy, comfortable carrying handles, big wide zipper for stuffing in massive loads. The "large" size holds 5,800 cubic inches, enough for anything but a monster pack, and sells for $49. The extra-large ($59) holds 8,830 cubic inches-enough for an expedition-sized pack, a case of beer, and a small child. The only drawback to the Bean bags is that they're so sturdily made they don't necessarily smash down into all that small a space.
More compressible, albeit less rugged, are duffels from Outdoor Products. That company's extra-large duffel is about the same size as the Bean large model, and sells for $30. You'll need to purchase an add-on shoulder harness for carrying it valise-style, but it does have D rings installed for that purpose.
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